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Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties
Materials Chemistry and Physics xxx (2013) 1e8
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Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties M.L. Keshtov a, Y. Arslan Udum c, L. Toppare b, d, e, *, V.S. Kochurov a, A.R. Khokhlov a a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, str. Vavilova 28, Moscow 119991, Russia Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey c Institute of Science and Technology, Department of Advanced Technologies, Gazi University, 06570 Ankara, Turkey d The Center for Solar Energy Research and Application (GÜNAM), Middle East Technical University, 06800 Ankara, Turkey e Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey b
h i g h l i g h t s < The polymer films have high transmissive color in the neutral state (0 V). < All of the polymers exhibit intense UV absorption and fluorescence both in DMF solutions and in film form. < The polymer films exhibit multi electrochromic properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2012 Received in revised form 12 February 2013 Accepted 16 February 2013
Synthesis of a series of new conjugated electrochromic polymeric pyridinium salts containing mainchain triphenylamine and their electrochromic properties were demonstrated. All polymers exhibit intense UV absorptions at 336e338 nm in DMF and 340e343 nm in thin film form and fluorescence centered at 410e438 nm in DMF and 460e461 nm in thin film form. The electrochromic properties of the films were investigated by electrochemical and spectroelectrochemical methods. Reversible redox signals with stable electrochromic characteristics were obtained via cyclic voltammetry. The electrochromic properties of the polymers remain highly stable after 50 cycles between 0 and 1.2 V. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochemical techniques Electrochemical properties Polymers Optical materials
1. Introduction In recent years, various inorganic and organic electrochromic materials have been intensively developed to improve dynamic and static electrochromic properties [1,2]. Conducting polymers possess substantial advantages over other materials owing to their better solubility and processibility and a quick response related to high conductivity and easy color adjustment implemented through variation in the length of p conjugation [3]. Recently, several n and p dopable electrochromic conducting polymers containing viologen fragments in side chains have been synthesized to attain the coordinated improvement of the contrast and multiplicity of color adjustment [4e7]. This method is useful when two electrochromic materials are incompatible and when it is
* Corresponding author. Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey. Tel.: þ90 3122103251; fax: þ90 3122103200. E-mail address: [email protected] (L. Toppare).
difficult to prepare homogeneous films from them. For example, it is difficult to prepare a homogeneous film from neutral poly (3,4-ethylene dioxythiophene) (PEDOT) and positively charged 4,40 -disubstituted bipyridyl salts, due to phase separation. Delongchamp et al. [8] successfully solved this problem through the layer-by-layer casting of PEDOTepolyviologen films [8]. However, this method has a substantial drawback related to the difficult reproduction of surface properties during preparation of the multilayer film and inevitable formation of charge traps at the interlayer joints. A comparison of the above-described methods reveals that, in the first, a high density of electrochromes cannot be achieved, because of crystallization, phase separation, and the development of concentration gradients. In the second method, these valuable physical properties are preserved; however, the quantum yield decreases noticeably. In order to overcome the above drawbacks and to further cooperatively improve the electrochromic characteristics, we first developed cathodeeanode complementary electrochromic conjugated polymers that are produced by incorporation of pyridinium salts and triarylamines
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
2
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
(TAAs) into macromolecular backbones. Such polymers exhibit a number of advantages since they make it possible to achieve an extremely high concentration of electrochromes in a polymer matrix without crystallization, phase separation, concentrationgradient development, and a decrease in the quantum yield. Polymeric pyridinium salts (PPSs) are suitable electrochromes of cathodic type since they possess a high thermal stability, stable color characteristics in three different oxidation states (V2, V1, V0), and a great modification potential to widen the range of useful service properties of electrochromes [9]. TAAs form stable cation radicals characterized by a high mobility of the charge and a long lifetime, which make them attractive electrochromic building blocks of the anode type [10e14]. Since the redox potentials of pyridinium salts and TAAs do not overlap, their complementary action improves the contrast and enriches the electrochromic colors relative to those of the pristine compounds. Aromatic PPSs are thermally stable. However, their rigid chains, strong hydrogen bonds, and high glass-transition temperatures (Tg) limit their solubility. The incorporation of bulky TAA units into PPSs not only increases their solubility without any loss in thermal stability but also imparts hole-transport and multi electrochromic properties to them. In order to obtain new electrochromic PPSs of both n and p dopable, we synthesized aromatic diamines containing triphenylamine and carbazole groups. 2. Experimental 2.1. Materials and methods The 1H and 13C NMR spectra of the initial compounds and the polymers were recorded on a Bruker AMX-400 spectrometer operating at frequencies of 400.13 and 100.62 MHz, respectively. The IR spectra were recorded with a PerkinElmer 1720-X FTIR spectrometer; TGA was performed with a PerkinElmer TGA-7 instrument at a heating rate of 20 K min1. The absorption spectra were measured within the range 190e900 nm on a Varian Cary 50 spectrophotometer. Colorimetry measurements were achieved by a Minolta CS-100A Chroma Meter with a 0/0 (normal/normal) viewing geometry as recommended by CIE. During measurement, samples were placed in a light booth system where it was illuminated from behind by a D65 light source. An L8253 xenon lamp was employed as a source of exciting radiation and was incorporated into a Hamamatsu LC-4 radiation unit equipped with a fiber-optic radiation output. Cyclic voltammetry was carried out under argon with an IPC PRO Econix potentiostategalvanostat using the standard three-electrode scheme. Test samples were prepared as films of the examined polymers applied onto glass substrates covered with ITO coatings (see the sample preparation for voltammetry measurements). Silver chloride and platinum electrodes were employed as reference and counter electrodes, respectively. The scanning was performed at a rate of 50 mV s1. The films of the examined polymers were prepared by the spin-coating method. Glasses covered with ITO coatings were used as substrates (a resistance of 6e15 U cm2). A polymer solution (10 mg ml1 in DMF) was applied onto a substrate mounted on a rotating (w1000 rpm) platform. The films were approximately 100 nm thick.
Then, the reaction mixture was cooled and poured into cold water (200 ml); the residue was filtered off and crystallized from acetic acid to obtain a white product. Yield, 10.21 g (65%), m.p. 192e144 C (lit. m.p. 194e196 C [7]). 1H NMR (400 MHz, DMSO): d 7.23 (d, 2H), 7.29 (m, 6H), 7.52 (t, 2H), 8.18 (d, 2H) ppm. Found (%): C, 64.30; H, 4.06; N, 8.49. For C18H17N3O4: Anal. Calcd (%): C, 64.48; H, 3.88; N, 8.36. 2.2.2. Compound Ib A mixture of NaH (3.00 g, 250 mmol) and DMSO (150 ml) was stirred at room temperature for 2 h. Then, diphenylamine (21.15 g, 125 mmol) and p-nitrofluorobenzene (17.64 g, 125 mmol) were added; the reaction mixture was heated at 100 C for 6 h and poured into water (2 l). The resulting precipitate was filtered off, dried, and crystallized from acetonitrile. Yield, 27.22 g (75%), m.p. 142e144 C (lit. m.p. 144e145 C [8]). 1H NMR (400 MHz, CDCl3): d 8.03 (d, 2H), 7.34e7.38 (m, 4H), 7.17e7.25 (m, 6H), 6.91 (d, 2H) ppm. Found (%): C, 74.58; H, 4.80; N, 9.72. For C18H14N2O2: Anal. Calcd (%): C, 74.47; H, 4.86; N, 9.65. 2.2.3. Compound Ic A 100-ml two-necked flask equipped with a reflux condenser and a stirrer was charged with carbazole (2.56 g, 15.3 mmol), K2CO3 (10.56 g, 76.52 mmol), p-nitrofluorobenzene (6.5 ml, 61.4 mmol), and DMF (80 ml). The reaction mixture was boiled for 12 h, cooled, and poured into water (500 ml). A yellow precipitate was filtered off, dried, and crystallized from benzene. Yield, 3.79 g (86%); m.p. 208e210 C (lit. m.p. 209e211 C [9]). 1H NMR (400 MHz, CDCl3): d 7.35 (t, 2H), 7.43e7.751 (m, 4H), 7.77 (d, 2H), 8.14 (d, 2H), 8.46 (d, 2H) ppm. Found (%): C, 74.96; H, 43.27; N, 9.85. For C18H12N2O2: Anal. Calcd (%): C, 74.99; H, 4.21; N, 9.72. 2.2.4. Compound IIb This compound was synthesized analogously to N-(4-aminophenyl)carbazole IIc. Amine IIb was obtained as yellow needles; yield, 2.179 g (91%); m.p. 148e150 C (m.p. 148e150 C [10]). IR (KBr): n 3438, 3355 cm1 (NeH). 1H NMR (400 MHz, CDCl3): d 7.21 (t, 4H), 7.05 (d, 4H), 6.98 (d, 2H), 6.93 (t, 2H), 6.65 (d, 2H), 3.51 (s, 2H) ppm. Found (%): C, 83.07; H, 6.09; N, 10.74. For C18H16N2: Anal. Calcd (%): C, 83.05; H, 6.20; N, 10.76. 2.2.5. Compound IIc A 50-ml three-necked flask equipped with a stirrer, a reflux condenser, and a dropping funnel was charged with N-(4nitrophenyl)carbazole (III) (2.595 g, 9 mmol), 10% Pd/C (0.09 g), and ethanol (27 ml), and the mixture was heated to boiling. Then, hydrazine monohydrate (2.3 ml) was slowly added dropwise, and the reaction mixture was boiled for 10 h. The mixture was cooled to room temperature, and the residual catalyst was filtered off. The filtrate was evaporated in a rotary evaporator to obtain yellow crystals. Yield, 2.07 g (89%), m.p. 91e93 C (lit. m.p. 91e93 C [10]). IR (KBr): n 3432, 3351 cm1 (NeH). 1H NMR (400 MHz, CDCl3): d 8.21 (d, 2H), 7.745e7.35 (m, 8H), 6.90 (d, 2H), 3.92 (s, 2H, NH2) ppm. Found (%): C, 83.30; H, 5.51; N, 10.75. For C18H14N2: Anal. Calcd (%): C, 83.69; H, 5.46; N, 10.84.
2.2. Synthesis 2.2.1. Compound Ia A mixture of freshly distilled aniline (4.34 g, 46.6 mmol), p-fluoronitrobenzene (13.70 g, 97.1 mmol), and calcium carbonate (8.50 g, 61.5 mmol) in DMSO (60 ml) was stirred at 130 C for 8 h.
2.2.6. Compound IIIb This compound was synthesized similarly to (4-nitrophenyl) diphenylamine I. Yield, 65%; m.p. 219e221 C; red crystals. 1H NMR (400 MHz, CDCl3): d 8.200 (d, 4H), 7.36 (t, 4H), 7.24 (d, 4H), 7.15 (d, 2H); 7.12 (t, 2H), 7.10 (d, 4H), 7.01(d, 2H) ppm.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
Found (%): C, 71.44; H, 4.45; N, 11.11. For C30H22N4O4: Anal. Calcd (%): C, 71.70; H, 4.42; N, 11.15. 2.2.7. Compound IIIc A 150-ml two-necked flask equipped with a stirrer and a reflux condenser was charged with NaH (2.95 g, 0.123 mol) and DMF (100 ml), and the mixture was stirred at room temperature for 1 h. Then, compound I (9.041 g 0.035 mol) and 4-fluoronitrobenzene (10.12 g, 0.072 mol) were simultaneously added, and the mixture was heated to 150 C under stirring for 10 h. The reaction mixture was cooled and poured into methanol (800 ml). The formed precipitate was filtered off, dried, and crystallized from DMF to obtain
3
red crystals. Yield, 65%; m.p. 282e282 C. 1H NMR (400 MHz, CDCl3): d 8.50 (d, 4H), 8.03 (d, 2H), 7.87 (d, 2H), 7.72 (m, 8H), 7.57 (m, 4H) ppm. Found (%): C, 72.01; H, 4.07; N, 11.35. For C30H20N4O4: Anal. Calcd (%): C, 71.99; H, 4.03; N, 11.19. 2.2.8. Compound IVa A 100-ml two-necked flask equipped with a reflux condenser and a stirrer was charged with 4,40 -dinitrotriphenylamine (3.262 g, 9.73 mmol), 10% Pd/C (0.108 g), hydrazine monohydrate (2.7 ml), and ethanol (81 ml); the mixture was boiled for 15 h and rapidly filtered to remove catalyst. The solvent was evaporated in a rotary
O 2N
N
NO2
NH 2
Ia
+
NH
F
NO2
NH2
N
N
Ib
IIb
NO2
NH2
N
N
NO2
NH
Ic
O2N
N
IIc
NO2
H 2N
Ia O2N
N
+
NO2
H2N
N
NH2
N
IIIb F
NH 2
IVa
N
IIb, IIc
N
IVb
NO2
O2N
N
NO2
H 2N
N
N
N
IIIc
IVc
NH2
Scheme 1.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
for 12 h to obtain a yellow powder. Yield, 95%; m.p. >350 C (decomposition).
evaporator. The crude product was crystallized from ethanol under argon and dried in vacuum at 70 C to obtain colorless needles. Yield, 2.44 g (91%); m.p. 187e189 C. IR (KBr): n 3422, 3351 cm1 (NeH). 1H NMR (CDCl3): d 7.13 (t, J ¼ 5.2 Hz, 2H), 6.94 (d, J ¼ 5.2, 4H), 6.89 (d, J ¼ 5.2, 2H), 6.79 (t, J ¼ 4.4, 1H), 6.61 (d, J ¼ 5.2, 4H), 3.78 (s, 4H) ppm. Found (%): C, 78.67; H, 6.19; N, 15.34. For C18H17N3: Anal. Calcd (%): C, 78.52; H, 6.22; N, 15.26. 2.2.9. Compound IVb This compound was synthesized similar to compound IVa. Yield, 3.31 g (87%). 1H NMR (400 MHz, DMSO): d 7.20 (t, 4H), 6.92 (d, 4H), 6.89 (t, 2H), 6.83 (d, 4H), 6.80 (d, 2H), 6.60 (d, 2H), 6.55 (d, 4H), 4.94 (s, NH2, 4H) ppm. Found (%): C, 81.53; H, 5.97; N, 12.64. For C30H26N4: Anal. Calcd (%): C, 81.14; H, 6.01; N, 12.42.
2.2.12. Poly(pyridinium triflate) VIa A 25-ml three-necked flask equipped with a stirrer, a reflux condenser, and an inlet for argon was charged with compound V (2.5165 g, 3 mmol), 4,40 -diaminotriphenylamine IVa (0.8261 g, 3 mmol), and DMSO (18 ml) and heated at 110 C for 2 h. Then, toluene (10 ml) was added. Azeotrope and excess toluene were distilled off from the reaction mixture at 150 N; then, the cooled solution was slowly poured into a large excess of diethyl ether. The precipitate was filtered off and dried at 120 C in vacuum for 24 h. The fiber like polymer was obtained in a quantitative yield. The reduced viscosity of polymer VIa was hred ¼ 4.5 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1625 (Ar), 1494 (CeN), 1270 (CeF) cm1. 1H NMR (400 MHz, DMSO): d 7.9e9.1 (6H, Ar), 6.8e8.2 (28H, Ar) ppm.
2.2.10. Compound IVc This compound was synthesized similar to IVa. Yield, 0.65 g (92%). 1H NMR (400 MHz, DMSO): d 8.12 (d, 2H), 7.40 (d, 4H), 7.24 (m, 4H), 7.05 (m, 6H), 6.66 (d, 4H), 4.93(s, NH2, 4H) ppm. Found (%): C, 81.73; H, 5.39; N, 12.64. For C30H24N4: Anal. Calcd (%): C, 81.79; H, 5.49; N, 12.72.
2.2.13. Poly(pyridinium triflate) VIb Poly(pyridinium triflate) VIb was synthesized similar to polymer VIa. The reduced viscosity of polymer VIb was hred ¼ 3.8 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1628(Ar), 1497 (CeN), 1273 (CeF) cm1. 1H NMR (400 MHz, DMSO): d 7.4e8.9 (22H, Ar), 6.8e8.2 (28H, Ar).
2.2.11. Compound V A 100-ml two-necked flask equipped with a stirrer and a reflux condenser was charged with terephthalic aldehyde (0.015 mol), acetophenone (0.09 mol), and ethanol (50 ml). The reaction mixture was heated to 70 C, and an aqueous KOH solution (2.1 ml, 0.0375 mol) was added dropwise for 30 min; the mixture was boiled for 5 h and cooled to room temperature. The precipitate was filtered off and washed with cold ethanol. The crude product was crystallized from toluene to obtain yellow crystals of 3,3-(p-phenylene)-bis(1,5-diphenyl-1,5-pentadienone). Yield, 98%; m.p. 205e 207 C, (lit. m.p. 205e207 C [6]). IR (KBr): n 1685 cm1 (C]O). Then, to a suspension of triphenylmethanol (20 mmol) and acetic anhydride (60 ml), at room temperature and under stirring, an aqueous 50% trifluoromethanesulfonic acid solution (0.025 mol) was added dropwise and the suspension was stirred for 1 h. Then, 3,3-(p-phenylene)-bis(1,5-diphenyl-1,5-pentadienone) (0.008 mol) was added and the reaction mixture was allowed to stand for a night. The precipitate was filtered off, washed with cold acetic anhydride and diethyl ether, and dried in vacuum at 80 C
2.2.14. Poly(pyridinium triflate) VIc Poly(pyridinium triflate) VIc was synthesized similar to polymer VIa. The reduced viscosity of polymer VIc was hred ¼ 4.1 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1632(Ar), 1495 (CeN), 1277 (CeF) cm1. 1 H NMR (400 MHz, DMSO): d 7.6e8.8 (20H, Ar), 6.7e8.3 (28H, Ar).
O
O
H2N
Ar
3. Results and discussion In order to obtain new electrochromic PPSs of both n and p dopable, we synthesized aromatic diamines containing triphenylamine and carbazole groups. These compounds were obtained via successive transformations comprising the interaction of diphenylamine and carbazole with an equimolar amount of p-nitrofluorobenzene under the conditions of nucleophilic substitution followed by a reduction of nitro derivatives I to intermediate amines II. The latter were, in turn, involved in the amination reaction with a twofold excess of p-nitrofluorobenzene, and the resulting 4,40 -bis(4-nitrophenyl)-N,N-diphenyl-1,4-phenyldiamine (IIIc) and 4,4-dinitro-4-N-carbozolyl-triphenylamine (IIIa) were
NH2
DMSO/toluene
N
Ar
(2)
2CF3SO3
2CF3SO3
V
where Ar =
N
VI N
(IVa),
N
N
(IVb) and
N
(IVc)
N
Scheme 2.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
5
Table 1 Some characteristics of electrochromic polymers VI. Polymer
ha (DMF, 25 C), dl g1
Tensile characteristics of the films
T10%, C
s, MP
3,
Air
Argon
VIa VIb VIc
4.5 3.8 4.1
50.0 41.9 46.6
5 4 6
460 445 450
470 460 465
a
%
Measured at a concentration of 0.1 g dl1.
reduced to desired diamines IV. The generalized scheme of the synthesis of these compounds is outlined below (Schemes 1 and 2). The composition and structure of intermediates IeIII and desired products IV were established by elemental analysis and IR and NMR spectroscopy (see Experimental). In particular, the IR spectra of compounds I and III comprise intense absorption bands at w1580 and w1320 cm1 attributed to NO2 groups. After reduction of nitro groups, these bands disappear in the spectra of compounds II and IV, while new bands characteristic of amino groups arise in the range 3300e3500 cm1. A series of new polymeric pyridinium salts VI were obtained through the interaction of equimolar amounts of bis(pyrilium triphthalate) V and aromatic diamines IV. PPSs were synthesized in DMSO at 120 C for 3 h; then, toluene was added to the reaction mixture to facilitate azeotropic distillation of water released during the reaction. Then, the reaction mixture was heated to 150 C and kept under these conditions for an additional 5 h. The process proceeded under homogeneous conditions, which allowed the synthesis of polymers with relatively high reduced viscosities of solutions (hred ¼ 3.8e4.5 dl g1). The structure of the polymers was studied by IR and 1H NMR spectroscopy. In particular, the IR spectra of the polymers comprise absorption bands with maxima at 1495 and 1275 cm1 assigned to the stretching vibrations of CeN bond and the stretching vibrations of CeF bonds of the triflate counter ion, while signals in the range 3200e2900 cm1 attributed to NH2 groups are absent in the spectra. Wide signals due to aromatic protons and pyridinium rings are observed in the 1H spectra at 7e8 and 8e9 ppm, respectively. Large chemical shifts correspond to protons located at positively charged nitrogen atoms. Signals at d ¼ 3e5 ppm characteristic of amino groups are absent, a fact that indicates the formation of a polymer with a low concentration of terminal groups. The thermal and thermooxidative stability of the polymers was investigated by DSC and TGA. Tg (below 400 C) was not observed in the DSC thermograms. It seems that the polymers undergo degradation before melting. The 10% mass loss temperatures of the polymeric pyridinium salts in air and argon lie at 400e460 and 411e470 C, respectively (Table 1). All polymers are soluble in polar organic solvents, such as DMSO, DMF, DMAA, and N-methylpyrrolidone. Their high solubility is likely related to the occurrence of the bulky side triphenyl groups and positive charges located along polymer chains. A good solubility in DMF and high viscosity characteristics of PPSs made it possible to
Fig. 1. Absorption and fluorescence spectra of polymers (1, 4) VIa, (2, 5) VIb, and (3, 6) VIc in (a) DMF (1.93 105 mol l1) and (b) thin films.
prepare strong elastic films, whose mechanical properties are listed in Table 1. As seen, the ultimate tensile strength and elongation at break are in the ranges 42e50 MPa and 4e6%, respectively. The optical and electrochemical properties of PPSs were studied by UV, visible, and photoluminescent spectroscopy and cyclic
Table 2 Some optical and electrochemical properties of polymers VI. abs fl Polymer lmax ; nm lmax ; nm lonset, nm HOMO, eV LUMO, eV Eopt, eV E1/2, V
VIa VIb VIc
343 336 340 338 343 338
460 438 461 410 460 438
392
5.49
2.33
3.16
1.13
395
5.07
1.93
3.14
0.71
392
5.28
2.12
3.16
0.92
Note: the numerators and denominators refer to the maximum light absorption and luminescence in thin films and DMF solutions (1.9 105 mol l1), respectively. Eopt ¼ 1240/lonset.
Fig. 2. Cyclic voltammograms of polymer film (VIa) measured in (a) water in the presence of 0.1 M KCl in the course of reduction and (b) acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation at a scan rate of 50 mV s1.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8 Table 3 Colorimetry studies of polymers VI. VIa
Fig. 3. Cyclic voltammogram of polymer film (VIb) in acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation scanning at a scan rate of 50 mV s1.
voltammetry. The spectral data are listed in Table 2, where corresponding absorption and fluorescence spectra of PPSs measured in solutions and thin films are demonstrated in Fig. 1. The maxima of the absorption spectra measured in DMF solutions and thin films are in the ranges 336e338 and 340e343 nm, respectively, while the maxima of the fluorescence spectra measured under the same conditions are observed at 410e438 and 460e461 nm. In a dilute solution, polymer chains are separated by solvent molecules, and the pep interaction between polymer chains is lower than in solid state. In a solid film, polymer chains are packed together, which results in the red shift of the UV/vis absorption spectrum. The electrochemical data are listed in Table 2 as well. All polymers have reversible or partly reversible redox properties owing to their high electrical activity. Cyclic voltammograms of polymers VI are presented in Figs. 2e4. Polymer VIa demonstrates one quasi-reversible redox pair at 1.13 V (Fig. 2b) which is assigned to cation radical R1 arising from elimination of an electron from a nitrogen atom of a triphenylamine fragment during oxidation in acetonitrile and two reversible redox pairs are assigned to a polymeric pyridine elements V1 and V2 during reduction in water (Fig. 2a). Polymer VIb exhibits two reversible redox pairs at 0.71
Fig. 4. Cyclic voltammogram of polymer film (VIb) in acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation and reduction scans at a scan rate of 50 mV s1.
VIb
VIc
0.0 V
1.3 V
0.0 V
1.2 V
0.0 V
1.4 V
L: 88.543 a: 6.675 b: 25.029
L: 62.844 a: 24.980 b: 9.855
L: 90.597 a: 5.513 b: 7.745
L: 70.106 a: 11.097 b: 11.347
L: 91.730 a: 8.085 b: 10.956
L: 70.940 a: 9.080 b: 2.231
and 1.12 eV which were attributed to the consecutive elimination of an electron from a nitrogen atom to yield stable monocation and dication radicals R1 and R2, respectively, upon oxidation (Figs. 3 and 4) and two irreversible redox pairs (the irreversible reduction of bipyridinium group may be due to its oxidation in air) assigned to polymeric pyridinium elements V2 and V1 during the reduction scan (Fig. 4, area of negative potential) in acetonitrile. Spectroelectrochemical studies of the polymer film revealed that the color of the polymer films changed during the p-doping
Fig. 5. Absorption spectra of polymer VIb in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) at different potentials: (a) (1) 0, (2) 0.20, (3) 0.5, (4) 0.71, and (5) 1.0, and (b) (1) 0, (2) 0.5, (3) 1, (4) 1.20, and (5) 1.30.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
+
N
+
N
7
N
n _
2 CF3 SO 3
N
- e+
N
e-
+
N
N
n
_
(3)
2 CF3 SO 3
.+ N
R1
- e-
+
N
e-
+
.+ N
N
n
_
2 CF3 SO 3
.+ N
R2 Scheme 3.
N
+
+
N
N
n
_
2 CF3 SO 3
N
V0
e N
.
+
e CF3 SO 3
N
N
n
_
(4)
N
V1
e e
N
.
.
N
N
n
V2
N
Scheme 4.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
Fig. 6. Stepwise-potential measurement of the absorption by polymer VIb during the stepped application of a voltage from 0 to 1.20 V (at a wavelength of 620 nm in acetonitrile containing 0.1 M tetrabutylammonium perchlorate).
(oxidation) process. The color changes were further investigated by colorimetry to define colors precisely. A polymer’s color is one of the most important properties for an application in display devices. For that reason, in situ colorimetric analysis of polymers was studied by colorimetry. Colorimetry analysis was used to make the measurement of the color in an objective and quantitative practice. The Commission Internationale de l’Eclairage (CIE) system was employed as the quantitative scale to define and compare colors. In the CIE system, luminance or brightness, hue and saturation, symbolized by L, a, b respectively, are determined to qualify color. The color coordinates, L, a, b values, were measured and summarized in Table 3. The band gap, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the polymers were given in Table 1. In particular, for polymer VIb, OX E we have: EHOMO ¼ EFc=Fcþ vac þ E1=2 1=2; Fc=Fcþ ¼ 5:07 eV; where EFc=Fcþ
vac
¼ 4:8 eV (the HOMOFc=Fcþ
vac
reference in vacuum),
OX ¼ 0:71 eV (polymer VIb), and E E1=2 1=2; Fc=Fcþ ¼ 0:44 eV (the
external redox reference in acetonitrile). The electrochromic properties of thin PPSs films were investigated with the help of an optically transparent thin electrode attached to a UVeVIS spectrometer. When the applied voltage to the film of polymer VIa is increased from 0 to 1.3 V, the signal attributed to cation radicals of polymeric pyridinium salt appears. The color of the films varies from the initial light yellow to green. At zero potential, the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. When the applied voltage to the film of polymer VIc is increased from 0 to 1.1particular, for polymer V the color of the films varies from the initial light yellow to light green. The signal is attributed to cation radicals of polymeric pyridinium salt. As the potential is further increased to 1.4particular, for polymer V the film becomes light blue. This signal is assigned to the dication of polymeric pyridinium salt. At zero potential the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. Typical electrochromic absorption spectra of polymer VIb and that were registered at different potentials are illustrated in Fig. 5. When
the applied voltage is increased from 0 to 1.0 V, a new peak arises at 850 nm and increases with a gain in potential. The new signal is attributed to cation radicals of polymeric pyridinium salt R1 (Fig. 5a); the color of the films varies from the initial light yellow to green. As the potential is further increased to 1.2 V, the characteristic peak at 850 nm gradually disappears with the concomitant appearance of a new band at 620 nm. This signal is assigned to the dication of polymeric pyridinium salt R2 (Fig. 5b); the film becomes blue. At zero potential, the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. The pathway of the anodic oxidation of polymer VIb is shown below (Scheme 3). When the potential was decreased from 0 to 1.20 V, polymer VIb was reduced to neutral violet form V2 and became colorless at zero potential V0 (Fig. 4). The cathodic reduction of polymer VIb occurs as follows (Scheme 4). The pathway of the anodic oxidation and reduction of polymer VIc is similar to the pathway of oxidation and reduction of polymer VIb. The time of color switching (coloration time) is assessed with the help of a stepped potential (Fig. 6) as the time required to achieve a 90% change in absorption after the potential is switched off. A thin film made of polymer VIb (lmax ¼ 620 nm) requires 3 s at 1.00 V for color switching and 2 s for bleaching. After ten cycles, the polymer still exhibits highly stable electrochemical characteristics (Fig. 6). 4. Conclusion A series of new conjugated electrochromic polymeric pyridinium salts containing triphenylamine units were synthesized and their electrochromism properties were demonstrated. The electrochromic properties of the films are investigated by electrochemical and spectroelectrochemical methods. The color changes were investigated for polymer films by colorimetry studies. The polymer films have high transmissive color in the neutral state. Reversible redox signals with stable electrochromic characteristics were found using cyclic voltammetry. The electrochromic properties of the polymers remain highly stable after 10 cycles in the range 0e1.2 V. All of the polymers exhibit intense UV absorption with maxima of absorption and fluorescence at 340e343 and 336e 338 nm in DMF solutions and 460e461 and 410e438 nm in films. The polymer films exhibit multi electrochromic properties. References [1] E.N. Esmer, S. Tarkuc, Y.A. Udum, L. Toppare, Mater. Chem. Phys. 131 (2011) 519e524. [2] M. Ak, L. Toppare, Mater. Chem. Phys. 114 (2009) 789e794. _ [3] A. Arslan, Ö. Türkarslan, C. Tanyeli, I.M. Akhmedov, L. Toppare, Mater. Chem. Phys. 104 (2007) 410e416. [4] H. Ko, M. Kang, B. Moon, H. Lee, Adv. Mater. 16 (2004) 1712e1716. [5] H. Ko, J. Yom, B. Moon, H. Lee, Electrochim. Acta 48 (2003) 4127e4135. [6] H. Ko, S. Park, W. Paik, H. Lee, Synth. Met. 132 (2002) 15e20. [7] H. Ko, S. Kim, H. Lee, B. Moon, Adv. Funct. Mater. 15 (2005) 905e909. [8] D. DeLongchamp, M. Kastantin, P. Hammond, Chem. Mater. 15 (2003) 1575e 1586. [9] F. Lin, S. Cheng, F. Harris, Polymer 43 (2002) 3421e3430. [10] G. Liou, S. Hsiao, Y. Fang, Eur. Polym. J. 42 (2006) 1533e1540. [11] G. Liou, S. Hsiao, N. Huang, Y. Yang, Macromolecules 39 (2006) 5337e5346. [12] Ch. Chang, G. Liou, S. Hsiao, J. Mater. Chem. 17 (2007) 1007e1015. [13] Y. Oishi, H. Takado, M. Yoneyama, J. Polym. Sci., Part A: Polym. Chem. 28 (1990) 1763e1769. [14] S. Cheng, S. Hsiao, T. Su, G. Liou, Macromolecules 38 (2005) 307e316.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties M.L. Keshtov a, Y. Arslan Udum c, L. Toppare b, d, e, *, V.S. Kochurov a, A.R. Khokhlov a a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, str. Vavilova 28, Moscow 119991, Russia Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey c Institute of Science and Technology, Department of Advanced Technologies, Gazi University, 06570 Ankara, Turkey d The Center for Solar Energy Research and Application (GÜNAM), Middle East Technical University, 06800 Ankara, Turkey e Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey b
h i g h l i g h t s < The polymer films have high transmissive color in the neutral state (0 V). < All of the polymers exhibit intense UV absorption and fluorescence both in DMF solutions and in film form. < The polymer films exhibit multi electrochromic properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2012 Received in revised form 12 February 2013 Accepted 16 February 2013
Synthesis of a series of new conjugated electrochromic polymeric pyridinium salts containing mainchain triphenylamine and their electrochromic properties were demonstrated. All polymers exhibit intense UV absorptions at 336e338 nm in DMF and 340e343 nm in thin film form and fluorescence centered at 410e438 nm in DMF and 460e461 nm in thin film form. The electrochromic properties of the films were investigated by electrochemical and spectroelectrochemical methods. Reversible redox signals with stable electrochromic characteristics were obtained via cyclic voltammetry. The electrochromic properties of the polymers remain highly stable after 50 cycles between 0 and 1.2 V. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochemical techniques Electrochemical properties Polymers Optical materials
1. Introduction In recent years, various inorganic and organic electrochromic materials have been intensively developed to improve dynamic and static electrochromic properties [1,2]. Conducting polymers possess substantial advantages over other materials owing to their better solubility and processibility and a quick response related to high conductivity and easy color adjustment implemented through variation in the length of p conjugation [3]. Recently, several n and p dopable electrochromic conducting polymers containing viologen fragments in side chains have been synthesized to attain the coordinated improvement of the contrast and multiplicity of color adjustment [4e7]. This method is useful when two electrochromic materials are incompatible and when it is
* Corresponding author. Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey. Tel.: þ90 3122103251; fax: þ90 3122103200. E-mail address: [email protected] (L. Toppare).
difficult to prepare homogeneous films from them. For example, it is difficult to prepare a homogeneous film from neutral poly (3,4-ethylene dioxythiophene) (PEDOT) and positively charged 4,40 -disubstituted bipyridyl salts, due to phase separation. Delongchamp et al. [8] successfully solved this problem through the layer-by-layer casting of PEDOTepolyviologen films [8]. However, this method has a substantial drawback related to the difficult reproduction of surface properties during preparation of the multilayer film and inevitable formation of charge traps at the interlayer joints. A comparison of the above-described methods reveals that, in the first, a high density of electrochromes cannot be achieved, because of crystallization, phase separation, and the development of concentration gradients. In the second method, these valuable physical properties are preserved; however, the quantum yield decreases noticeably. In order to overcome the above drawbacks and to further cooperatively improve the electrochromic characteristics, we first developed cathodeeanode complementary electrochromic conjugated polymers that are produced by incorporation of pyridinium salts and triarylamines
0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
(TAAs) into macromolecular backbones. Such polymers exhibit a number of advantages since they make it possible to achieve an extremely high concentration of electrochromes in a polymer matrix without crystallization, phase separation, concentrationgradient development, and a decrease in the quantum yield. Polymeric pyridinium salts (PPSs) are suitable electrochromes of cathodic type since they possess a high thermal stability, stable color characteristics in three different oxidation states (V2, V1, V0), and a great modification potential to widen the range of useful service properties of electrochromes [9]. TAAs form stable cation radicals characterized by a high mobility of the charge and a long lifetime, which make them attractive electrochromic building blocks of the anode type [10e14]. Since the redox potentials of pyridinium salts and TAAs do not overlap, their complementary action improves the contrast and enriches the electrochromic colors relative to those of the pristine compounds. Aromatic PPSs are thermally stable. However, their rigid chains, strong hydrogen bonds, and high glass-transition temperatures (Tg) limit their solubility. The incorporation of bulky TAA units into PPSs not only increases their solubility without any loss in thermal stability but also imparts hole-transport and multi electrochromic properties to them. In order to obtain new electrochromic PPSs of both n and p dopable, we synthesized aromatic diamines containing triphenylamine and carbazole groups. 2. Experimental 2.1. Materials and methods The 1H and 13C NMR spectra of the initial compounds and the polymers were recorded on a Bruker AMX-400 spectrometer operating at frequencies of 400.13 and 100.62 MHz, respectively. The IR spectra were recorded with a PerkinElmer 1720-X FTIR spectrometer; TGA was performed with a PerkinElmer TGA-7 instrument at a heating rate of 20 K min1. The absorption spectra were measured within the range 190e900 nm on a Varian Cary 50 spectrophotometer. Colorimetry measurements were achieved by a Minolta CS-100A Chroma Meter with a 0/0 (normal/normal) viewing geometry as recommended by CIE. During measurement, samples were placed in a light booth system where it was illuminated from behind by a D65 light source. An L8253 xenon lamp was employed as a source of exciting radiation and was incorporated into a Hamamatsu LC-4 radiation unit equipped with a fiber-optic radiation output. Cyclic voltammetry was carried out under argon with an IPC PRO Econix potentiostategalvanostat using the standard three-electrode scheme. Test samples were prepared as films of the examined polymers applied onto glass substrates covered with ITO coatings (see the sample preparation for voltammetry measurements). Silver chloride and platinum electrodes were employed as reference and counter electrodes, respectively. The scanning was performed at a rate of 50 mV s1. The films of the examined polymers were prepared by the spin-coating method. Glasses covered with ITO coatings were used as substrates (a resistance of 6e15 U cm2). A polymer solution (10 mg ml1 in DMF) was applied onto a substrate mounted on a rotating (w1000 rpm) platform. The films were approximately 100 nm thick.
Then, the reaction mixture was cooled and poured into cold water (200 ml); the residue was filtered off and crystallized from acetic acid to obtain a white product. Yield, 10.21 g (65%), m.p. 192e144 C (lit. m.p. 194e196 C [7]). 1H NMR (400 MHz, DMSO): d 7.23 (d, 2H), 7.29 (m, 6H), 7.52 (t, 2H), 8.18 (d, 2H) ppm. Found (%): C, 64.30; H, 4.06; N, 8.49. For C18H17N3O4: Anal. Calcd (%): C, 64.48; H, 3.88; N, 8.36. 2.2.2. Compound Ib A mixture of NaH (3.00 g, 250 mmol) and DMSO (150 ml) was stirred at room temperature for 2 h. Then, diphenylamine (21.15 g, 125 mmol) and p-nitrofluorobenzene (17.64 g, 125 mmol) were added; the reaction mixture was heated at 100 C for 6 h and poured into water (2 l). The resulting precipitate was filtered off, dried, and crystallized from acetonitrile. Yield, 27.22 g (75%), m.p. 142e144 C (lit. m.p. 144e145 C [8]). 1H NMR (400 MHz, CDCl3): d 8.03 (d, 2H), 7.34e7.38 (m, 4H), 7.17e7.25 (m, 6H), 6.91 (d, 2H) ppm. Found (%): C, 74.58; H, 4.80; N, 9.72. For C18H14N2O2: Anal. Calcd (%): C, 74.47; H, 4.86; N, 9.65. 2.2.3. Compound Ic A 100-ml two-necked flask equipped with a reflux condenser and a stirrer was charged with carbazole (2.56 g, 15.3 mmol), K2CO3 (10.56 g, 76.52 mmol), p-nitrofluorobenzene (6.5 ml, 61.4 mmol), and DMF (80 ml). The reaction mixture was boiled for 12 h, cooled, and poured into water (500 ml). A yellow precipitate was filtered off, dried, and crystallized from benzene. Yield, 3.79 g (86%); m.p. 208e210 C (lit. m.p. 209e211 C [9]). 1H NMR (400 MHz, CDCl3): d 7.35 (t, 2H), 7.43e7.751 (m, 4H), 7.77 (d, 2H), 8.14 (d, 2H), 8.46 (d, 2H) ppm. Found (%): C, 74.96; H, 43.27; N, 9.85. For C18H12N2O2: Anal. Calcd (%): C, 74.99; H, 4.21; N, 9.72. 2.2.4. Compound IIb This compound was synthesized analogously to N-(4-aminophenyl)carbazole IIc. Amine IIb was obtained as yellow needles; yield, 2.179 g (91%); m.p. 148e150 C (m.p. 148e150 C [10]). IR (KBr): n 3438, 3355 cm1 (NeH). 1H NMR (400 MHz, CDCl3): d 7.21 (t, 4H), 7.05 (d, 4H), 6.98 (d, 2H), 6.93 (t, 2H), 6.65 (d, 2H), 3.51 (s, 2H) ppm. Found (%): C, 83.07; H, 6.09; N, 10.74. For C18H16N2: Anal. Calcd (%): C, 83.05; H, 6.20; N, 10.76. 2.2.5. Compound IIc A 50-ml three-necked flask equipped with a stirrer, a reflux condenser, and a dropping funnel was charged with N-(4nitrophenyl)carbazole (III) (2.595 g, 9 mmol), 10% Pd/C (0.09 g), and ethanol (27 ml), and the mixture was heated to boiling. Then, hydrazine monohydrate (2.3 ml) was slowly added dropwise, and the reaction mixture was boiled for 10 h. The mixture was cooled to room temperature, and the residual catalyst was filtered off. The filtrate was evaporated in a rotary evaporator to obtain yellow crystals. Yield, 2.07 g (89%), m.p. 91e93 C (lit. m.p. 91e93 C [10]). IR (KBr): n 3432, 3351 cm1 (NeH). 1H NMR (400 MHz, CDCl3): d 8.21 (d, 2H), 7.745e7.35 (m, 8H), 6.90 (d, 2H), 3.92 (s, 2H, NH2) ppm. Found (%): C, 83.30; H, 5.51; N, 10.75. For C18H14N2: Anal. Calcd (%): C, 83.69; H, 5.46; N, 10.84.
2.2. Synthesis 2.2.1. Compound Ia A mixture of freshly distilled aniline (4.34 g, 46.6 mmol), p-fluoronitrobenzene (13.70 g, 97.1 mmol), and calcium carbonate (8.50 g, 61.5 mmol) in DMSO (60 ml) was stirred at 130 C for 8 h.
2.2.6. Compound IIIb This compound was synthesized similarly to (4-nitrophenyl) diphenylamine I. Yield, 65%; m.p. 219e221 C; red crystals. 1H NMR (400 MHz, CDCl3): d 8.200 (d, 4H), 7.36 (t, 4H), 7.24 (d, 4H), 7.15 (d, 2H); 7.12 (t, 2H), 7.10 (d, 4H), 7.01(d, 2H) ppm.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
Found (%): C, 71.44; H, 4.45; N, 11.11. For C30H22N4O4: Anal. Calcd (%): C, 71.70; H, 4.42; N, 11.15. 2.2.7. Compound IIIc A 150-ml two-necked flask equipped with a stirrer and a reflux condenser was charged with NaH (2.95 g, 0.123 mol) and DMF (100 ml), and the mixture was stirred at room temperature for 1 h. Then, compound I (9.041 g 0.035 mol) and 4-fluoronitrobenzene (10.12 g, 0.072 mol) were simultaneously added, and the mixture was heated to 150 C under stirring for 10 h. The reaction mixture was cooled and poured into methanol (800 ml). The formed precipitate was filtered off, dried, and crystallized from DMF to obtain
3
red crystals. Yield, 65%; m.p. 282e282 C. 1H NMR (400 MHz, CDCl3): d 8.50 (d, 4H), 8.03 (d, 2H), 7.87 (d, 2H), 7.72 (m, 8H), 7.57 (m, 4H) ppm. Found (%): C, 72.01; H, 4.07; N, 11.35. For C30H20N4O4: Anal. Calcd (%): C, 71.99; H, 4.03; N, 11.19. 2.2.8. Compound IVa A 100-ml two-necked flask equipped with a reflux condenser and a stirrer was charged with 4,40 -dinitrotriphenylamine (3.262 g, 9.73 mmol), 10% Pd/C (0.108 g), hydrazine monohydrate (2.7 ml), and ethanol (81 ml); the mixture was boiled for 15 h and rapidly filtered to remove catalyst. The solvent was evaporated in a rotary
O 2N
N
NO2
NH 2
Ia
+
NH
F
NO2
NH2
N
N
Ib
IIb
NO2
NH2
N
N
NO2
NH
Ic
O2N
N
IIc
NO2
H 2N
Ia O2N
N
+
NO2
H2N
N
NH2
N
IIIb F
NH 2
IVa
N
IIb, IIc
N
IVb
NO2
O2N
N
NO2
H 2N
N
N
N
IIIc
IVc
NH2
Scheme 1.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
for 12 h to obtain a yellow powder. Yield, 95%; m.p. >350 C (decomposition).
evaporator. The crude product was crystallized from ethanol under argon and dried in vacuum at 70 C to obtain colorless needles. Yield, 2.44 g (91%); m.p. 187e189 C. IR (KBr): n 3422, 3351 cm1 (NeH). 1H NMR (CDCl3): d 7.13 (t, J ¼ 5.2 Hz, 2H), 6.94 (d, J ¼ 5.2, 4H), 6.89 (d, J ¼ 5.2, 2H), 6.79 (t, J ¼ 4.4, 1H), 6.61 (d, J ¼ 5.2, 4H), 3.78 (s, 4H) ppm. Found (%): C, 78.67; H, 6.19; N, 15.34. For C18H17N3: Anal. Calcd (%): C, 78.52; H, 6.22; N, 15.26. 2.2.9. Compound IVb This compound was synthesized similar to compound IVa. Yield, 3.31 g (87%). 1H NMR (400 MHz, DMSO): d 7.20 (t, 4H), 6.92 (d, 4H), 6.89 (t, 2H), 6.83 (d, 4H), 6.80 (d, 2H), 6.60 (d, 2H), 6.55 (d, 4H), 4.94 (s, NH2, 4H) ppm. Found (%): C, 81.53; H, 5.97; N, 12.64. For C30H26N4: Anal. Calcd (%): C, 81.14; H, 6.01; N, 12.42.
2.2.12. Poly(pyridinium triflate) VIa A 25-ml three-necked flask equipped with a stirrer, a reflux condenser, and an inlet for argon was charged with compound V (2.5165 g, 3 mmol), 4,40 -diaminotriphenylamine IVa (0.8261 g, 3 mmol), and DMSO (18 ml) and heated at 110 C for 2 h. Then, toluene (10 ml) was added. Azeotrope and excess toluene were distilled off from the reaction mixture at 150 N; then, the cooled solution was slowly poured into a large excess of diethyl ether. The precipitate was filtered off and dried at 120 C in vacuum for 24 h. The fiber like polymer was obtained in a quantitative yield. The reduced viscosity of polymer VIa was hred ¼ 4.5 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1625 (Ar), 1494 (CeN), 1270 (CeF) cm1. 1H NMR (400 MHz, DMSO): d 7.9e9.1 (6H, Ar), 6.8e8.2 (28H, Ar) ppm.
2.2.10. Compound IVc This compound was synthesized similar to IVa. Yield, 0.65 g (92%). 1H NMR (400 MHz, DMSO): d 8.12 (d, 2H), 7.40 (d, 4H), 7.24 (m, 4H), 7.05 (m, 6H), 6.66 (d, 4H), 4.93(s, NH2, 4H) ppm. Found (%): C, 81.73; H, 5.39; N, 12.64. For C30H24N4: Anal. Calcd (%): C, 81.79; H, 5.49; N, 12.72.
2.2.13. Poly(pyridinium triflate) VIb Poly(pyridinium triflate) VIb was synthesized similar to polymer VIa. The reduced viscosity of polymer VIb was hred ¼ 3.8 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1628(Ar), 1497 (CeN), 1273 (CeF) cm1. 1H NMR (400 MHz, DMSO): d 7.4e8.9 (22H, Ar), 6.8e8.2 (28H, Ar).
2.2.11. Compound V A 100-ml two-necked flask equipped with a stirrer and a reflux condenser was charged with terephthalic aldehyde (0.015 mol), acetophenone (0.09 mol), and ethanol (50 ml). The reaction mixture was heated to 70 C, and an aqueous KOH solution (2.1 ml, 0.0375 mol) was added dropwise for 30 min; the mixture was boiled for 5 h and cooled to room temperature. The precipitate was filtered off and washed with cold ethanol. The crude product was crystallized from toluene to obtain yellow crystals of 3,3-(p-phenylene)-bis(1,5-diphenyl-1,5-pentadienone). Yield, 98%; m.p. 205e 207 C, (lit. m.p. 205e207 C [6]). IR (KBr): n 1685 cm1 (C]O). Then, to a suspension of triphenylmethanol (20 mmol) and acetic anhydride (60 ml), at room temperature and under stirring, an aqueous 50% trifluoromethanesulfonic acid solution (0.025 mol) was added dropwise and the suspension was stirred for 1 h. Then, 3,3-(p-phenylene)-bis(1,5-diphenyl-1,5-pentadienone) (0.008 mol) was added and the reaction mixture was allowed to stand for a night. The precipitate was filtered off, washed with cold acetic anhydride and diethyl ether, and dried in vacuum at 80 C
2.2.14. Poly(pyridinium triflate) VIc Poly(pyridinium triflate) VIc was synthesized similar to polymer VIa. The reduced viscosity of polymer VIc was hred ¼ 4.1 dl g1 (DMF, 25 C, 0.1 g dl1). IR (KBr): n 1632(Ar), 1495 (CeN), 1277 (CeF) cm1. 1 H NMR (400 MHz, DMSO): d 7.6e8.8 (20H, Ar), 6.7e8.3 (28H, Ar).
O
O
H2N
Ar
3. Results and discussion In order to obtain new electrochromic PPSs of both n and p dopable, we synthesized aromatic diamines containing triphenylamine and carbazole groups. These compounds were obtained via successive transformations comprising the interaction of diphenylamine and carbazole with an equimolar amount of p-nitrofluorobenzene under the conditions of nucleophilic substitution followed by a reduction of nitro derivatives I to intermediate amines II. The latter were, in turn, involved in the amination reaction with a twofold excess of p-nitrofluorobenzene, and the resulting 4,40 -bis(4-nitrophenyl)-N,N-diphenyl-1,4-phenyldiamine (IIIc) and 4,4-dinitro-4-N-carbozolyl-triphenylamine (IIIa) were
NH2
DMSO/toluene
N
Ar
(2)
2CF3SO3
2CF3SO3
V
where Ar =
N
VI N
(IVa),
N
N
(IVb) and
N
(IVc)
N
Scheme 2.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
5
Table 1 Some characteristics of electrochromic polymers VI. Polymer
ha (DMF, 25 C), dl g1
Tensile characteristics of the films
T10%, C
s, MP
3,
Air
Argon
VIa VIb VIc
4.5 3.8 4.1
50.0 41.9 46.6
5 4 6
460 445 450
470 460 465
a
%
Measured at a concentration of 0.1 g dl1.
reduced to desired diamines IV. The generalized scheme of the synthesis of these compounds is outlined below (Schemes 1 and 2). The composition and structure of intermediates IeIII and desired products IV were established by elemental analysis and IR and NMR spectroscopy (see Experimental). In particular, the IR spectra of compounds I and III comprise intense absorption bands at w1580 and w1320 cm1 attributed to NO2 groups. After reduction of nitro groups, these bands disappear in the spectra of compounds II and IV, while new bands characteristic of amino groups arise in the range 3300e3500 cm1. A series of new polymeric pyridinium salts VI were obtained through the interaction of equimolar amounts of bis(pyrilium triphthalate) V and aromatic diamines IV. PPSs were synthesized in DMSO at 120 C for 3 h; then, toluene was added to the reaction mixture to facilitate azeotropic distillation of water released during the reaction. Then, the reaction mixture was heated to 150 C and kept under these conditions for an additional 5 h. The process proceeded under homogeneous conditions, which allowed the synthesis of polymers with relatively high reduced viscosities of solutions (hred ¼ 3.8e4.5 dl g1). The structure of the polymers was studied by IR and 1H NMR spectroscopy. In particular, the IR spectra of the polymers comprise absorption bands with maxima at 1495 and 1275 cm1 assigned to the stretching vibrations of CeN bond and the stretching vibrations of CeF bonds of the triflate counter ion, while signals in the range 3200e2900 cm1 attributed to NH2 groups are absent in the spectra. Wide signals due to aromatic protons and pyridinium rings are observed in the 1H spectra at 7e8 and 8e9 ppm, respectively. Large chemical shifts correspond to protons located at positively charged nitrogen atoms. Signals at d ¼ 3e5 ppm characteristic of amino groups are absent, a fact that indicates the formation of a polymer with a low concentration of terminal groups. The thermal and thermooxidative stability of the polymers was investigated by DSC and TGA. Tg (below 400 C) was not observed in the DSC thermograms. It seems that the polymers undergo degradation before melting. The 10% mass loss temperatures of the polymeric pyridinium salts in air and argon lie at 400e460 and 411e470 C, respectively (Table 1). All polymers are soluble in polar organic solvents, such as DMSO, DMF, DMAA, and N-methylpyrrolidone. Their high solubility is likely related to the occurrence of the bulky side triphenyl groups and positive charges located along polymer chains. A good solubility in DMF and high viscosity characteristics of PPSs made it possible to
Fig. 1. Absorption and fluorescence spectra of polymers (1, 4) VIa, (2, 5) VIb, and (3, 6) VIc in (a) DMF (1.93 105 mol l1) and (b) thin films.
prepare strong elastic films, whose mechanical properties are listed in Table 1. As seen, the ultimate tensile strength and elongation at break are in the ranges 42e50 MPa and 4e6%, respectively. The optical and electrochemical properties of PPSs were studied by UV, visible, and photoluminescent spectroscopy and cyclic
Table 2 Some optical and electrochemical properties of polymers VI. abs fl Polymer lmax ; nm lmax ; nm lonset, nm HOMO, eV LUMO, eV Eopt, eV E1/2, V
VIa VIb VIc
343 336 340 338 343 338
460 438 461 410 460 438
392
5.49
2.33
3.16
1.13
395
5.07
1.93
3.14
0.71
392
5.28
2.12
3.16
0.92
Note: the numerators and denominators refer to the maximum light absorption and luminescence in thin films and DMF solutions (1.9 105 mol l1), respectively. Eopt ¼ 1240/lonset.
Fig. 2. Cyclic voltammograms of polymer film (VIa) measured in (a) water in the presence of 0.1 M KCl in the course of reduction and (b) acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation at a scan rate of 50 mV s1.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
6
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8 Table 3 Colorimetry studies of polymers VI. VIa
Fig. 3. Cyclic voltammogram of polymer film (VIb) in acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation scanning at a scan rate of 50 mV s1.
voltammetry. The spectral data are listed in Table 2, where corresponding absorption and fluorescence spectra of PPSs measured in solutions and thin films are demonstrated in Fig. 1. The maxima of the absorption spectra measured in DMF solutions and thin films are in the ranges 336e338 and 340e343 nm, respectively, while the maxima of the fluorescence spectra measured under the same conditions are observed at 410e438 and 460e461 nm. In a dilute solution, polymer chains are separated by solvent molecules, and the pep interaction between polymer chains is lower than in solid state. In a solid film, polymer chains are packed together, which results in the red shift of the UV/vis absorption spectrum. The electrochemical data are listed in Table 2 as well. All polymers have reversible or partly reversible redox properties owing to their high electrical activity. Cyclic voltammograms of polymers VI are presented in Figs. 2e4. Polymer VIa demonstrates one quasi-reversible redox pair at 1.13 V (Fig. 2b) which is assigned to cation radical R1 arising from elimination of an electron from a nitrogen atom of a triphenylamine fragment during oxidation in acetonitrile and two reversible redox pairs are assigned to a polymeric pyridine elements V1 and V2 during reduction in water (Fig. 2a). Polymer VIb exhibits two reversible redox pairs at 0.71
Fig. 4. Cyclic voltammogram of polymer film (VIb) in acetonitrile in the presence of 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) in the course of oxidation and reduction scans at a scan rate of 50 mV s1.
VIb
VIc
0.0 V
1.3 V
0.0 V
1.2 V
0.0 V
1.4 V
L: 88.543 a: 6.675 b: 25.029
L: 62.844 a: 24.980 b: 9.855
L: 90.597 a: 5.513 b: 7.745
L: 70.106 a: 11.097 b: 11.347
L: 91.730 a: 8.085 b: 10.956
L: 70.940 a: 9.080 b: 2.231
and 1.12 eV which were attributed to the consecutive elimination of an electron from a nitrogen atom to yield stable monocation and dication radicals R1 and R2, respectively, upon oxidation (Figs. 3 and 4) and two irreversible redox pairs (the irreversible reduction of bipyridinium group may be due to its oxidation in air) assigned to polymeric pyridinium elements V2 and V1 during the reduction scan (Fig. 4, area of negative potential) in acetonitrile. Spectroelectrochemical studies of the polymer film revealed that the color of the polymer films changed during the p-doping
Fig. 5. Absorption spectra of polymer VIb in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (relative to Ag/AgCl) at different potentials: (a) (1) 0, (2) 0.20, (3) 0.5, (4) 0.71, and (5) 1.0, and (b) (1) 0, (2) 0.5, (3) 1, (4) 1.20, and (5) 1.30.
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M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
+
N
+
N
7
N
n _
2 CF3 SO 3
N
- e+
N
e-
+
N
N
n
_
(3)
2 CF3 SO 3
.+ N
R1
- e-
+
N
e-
+
.+ N
N
n
_
2 CF3 SO 3
.+ N
R2 Scheme 3.
N
+
+
N
N
n
_
2 CF3 SO 3
N
V0
e N
.
+
e CF3 SO 3
N
N
n
_
(4)
N
V1
e e
N
.
.
N
N
n
V2
N
Scheme 4.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059
8
M.L. Keshtov et al. / Materials Chemistry and Physics xxx (2013) 1e8
Fig. 6. Stepwise-potential measurement of the absorption by polymer VIb during the stepped application of a voltage from 0 to 1.20 V (at a wavelength of 620 nm in acetonitrile containing 0.1 M tetrabutylammonium perchlorate).
(oxidation) process. The color changes were further investigated by colorimetry to define colors precisely. A polymer’s color is one of the most important properties for an application in display devices. For that reason, in situ colorimetric analysis of polymers was studied by colorimetry. Colorimetry analysis was used to make the measurement of the color in an objective and quantitative practice. The Commission Internationale de l’Eclairage (CIE) system was employed as the quantitative scale to define and compare colors. In the CIE system, luminance or brightness, hue and saturation, symbolized by L, a, b respectively, are determined to qualify color. The color coordinates, L, a, b values, were measured and summarized in Table 3. The band gap, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the polymers were given in Table 1. In particular, for polymer VIb, OX E we have: EHOMO ¼ EFc=Fcþ vac þ E1=2 1=2; Fc=Fcþ ¼ 5:07 eV; where EFc=Fcþ
vac
¼ 4:8 eV (the HOMOFc=Fcþ
vac
reference in vacuum),
OX ¼ 0:71 eV (polymer VIb), and E E1=2 1=2; Fc=Fcþ ¼ 0:44 eV (the
external redox reference in acetonitrile). The electrochromic properties of thin PPSs films were investigated with the help of an optically transparent thin electrode attached to a UVeVIS spectrometer. When the applied voltage to the film of polymer VIa is increased from 0 to 1.3 V, the signal attributed to cation radicals of polymeric pyridinium salt appears. The color of the films varies from the initial light yellow to green. At zero potential, the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. When the applied voltage to the film of polymer VIc is increased from 0 to 1.1particular, for polymer V the color of the films varies from the initial light yellow to light green. The signal is attributed to cation radicals of polymeric pyridinium salt. As the potential is further increased to 1.4particular, for polymer V the film becomes light blue. This signal is assigned to the dication of polymeric pyridinium salt. At zero potential the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. Typical electrochromic absorption spectra of polymer VIb and that were registered at different potentials are illustrated in Fig. 5. When
the applied voltage is increased from 0 to 1.0 V, a new peak arises at 850 nm and increases with a gain in potential. The new signal is attributed to cation radicals of polymeric pyridinium salt R1 (Fig. 5a); the color of the films varies from the initial light yellow to green. As the potential is further increased to 1.2 V, the characteristic peak at 850 nm gradually disappears with the concomitant appearance of a new band at 620 nm. This signal is assigned to the dication of polymeric pyridinium salt R2 (Fig. 5b); the film becomes blue. At zero potential, the polymer becomes colorless and the intensity of its absorption spectrum becomes nil. The pathway of the anodic oxidation of polymer VIb is shown below (Scheme 3). When the potential was decreased from 0 to 1.20 V, polymer VIb was reduced to neutral violet form V2 and became colorless at zero potential V0 (Fig. 4). The cathodic reduction of polymer VIb occurs as follows (Scheme 4). The pathway of the anodic oxidation and reduction of polymer VIc is similar to the pathway of oxidation and reduction of polymer VIb. The time of color switching (coloration time) is assessed with the help of a stepped potential (Fig. 6) as the time required to achieve a 90% change in absorption after the potential is switched off. A thin film made of polymer VIb (lmax ¼ 620 nm) requires 3 s at 1.00 V for color switching and 2 s for bleaching. After ten cycles, the polymer still exhibits highly stable electrochemical characteristics (Fig. 6). 4. Conclusion A series of new conjugated electrochromic polymeric pyridinium salts containing triphenylamine units were synthesized and their electrochromism properties were demonstrated. The electrochromic properties of the films are investigated by electrochemical and spectroelectrochemical methods. The color changes were investigated for polymer films by colorimetry studies. The polymer films have high transmissive color in the neutral state. Reversible redox signals with stable electrochromic characteristics were found using cyclic voltammetry. The electrochromic properties of the polymers remain highly stable after 10 cycles in the range 0e1.2 V. All of the polymers exhibit intense UV absorption with maxima of absorption and fluorescence at 340e343 and 336e 338 nm in DMF solutions and 460e461 and 410e438 nm in films. The polymer films exhibit multi electrochromic properties. References [1] E.N. Esmer, S. Tarkuc, Y.A. Udum, L. Toppare, Mater. Chem. Phys. 131 (2011) 519e524. [2] M. Ak, L. Toppare, Mater. Chem. Phys. 114 (2009) 789e794. _ [3] A. Arslan, Ö. Türkarslan, C. Tanyeli, I.M. Akhmedov, L. Toppare, Mater. Chem. Phys. 104 (2007) 410e416. [4] H. Ko, M. Kang, B. Moon, H. Lee, Adv. Mater. 16 (2004) 1712e1716. [5] H. Ko, J. Yom, B. Moon, H. Lee, Electrochim. Acta 48 (2003) 4127e4135. [6] H. Ko, S. Park, W. Paik, H. Lee, Synth. Met. 132 (2002) 15e20. [7] H. Ko, S. Kim, H. Lee, B. Moon, Adv. Funct. Mater. 15 (2005) 905e909. [8] D. DeLongchamp, M. Kastantin, P. Hammond, Chem. Mater. 15 (2003) 1575e 1586. [9] F. Lin, S. Cheng, F. Harris, Polymer 43 (2002) 3421e3430. [10] G. Liou, S. Hsiao, Y. Fang, Eur. Polym. J. 42 (2006) 1533e1540. [11] G. Liou, S. Hsiao, N. Huang, Y. Yang, Macromolecules 39 (2006) 5337e5346. [12] Ch. Chang, G. Liou, S. Hsiao, J. Mater. Chem. 17 (2007) 1007e1015. [13] Y. Oishi, H. Takado, M. Yoneyama, J. Polym. Sci., Part A: Polym. Chem. 28 (1990) 1763e1769. [14] S. Cheng, S. Hsiao, T. Su, G. Liou, Macromolecules 38 (2005) 307e316.
Please cite this article in press as: M.L. Keshtov, et al., Synthesis of aromatic poly(pyridinium salt)s and their electrochromic properties, Materials Chemistry and Physics (2013), http://dx.doi.org/10.1016/j.matchemphys.2013.02.059